Many modern radio frequency (RF) base stations have separate RF and control sections. A base station RF section typically includes mostly analog circuitry (e.g., RF amplifiers, RF oscillators, RF and IF receiver amplifiers/filters, etc.) Most modern RF base stations available today control such an RF section with a digital (e.g., microprocessor based) control section. Such a digital control section can provide expanded control capabilities and flexibility, thereby providing many advantages over prior analog control circuit arrangements.
As is well known, digital circuits are not particularly compatible with RF signals. The RF section of a base station is therefore almost always housed in a separate shielded enclosure to prevent RF signals radiated directly by the power amplifier and other components therein from reaching the sensitive digital circuitry and vice versa (digital circuitry getting into receiver).
Interconnecting the RF section with the control section is typically a relatively difficult and significant problem. A relatively large number of signals must pass between the RF section and the control section. For example, the control section may typically program the operating frequencies of the transmitter and the receiver within the RF section and may also directly control various other parameters of RF section operation (e.g., whether or not the transmitter is keyed, the state of an RF antenna relay, the transmitter final amplifier power output, etc.). In addition, the control section must monitor various status and other parameters provided by the RF section in order to ascertain the state of operation of the RF section. For example, the control section may monitor received signal strength and/or carrier detect, DC input current to the transmitter final amplifier, etc.).
In the past, such control and status signals have been communicated between the control section and the RF section over an array of dedicated parallel conductors. One typical configuration used in the past was to provide the RF section and the control section each with a multipin connector. A multiconductor cable (or, in some cases, a bus backplane) was used to convey, in parallel, all of the various signals that needed to be communicated between the RF section and the control section. For example, one or more conductors might be dedicated to carrying a signal generated by the control section for controlling whether or not the transmitter in the RF section is keyed; one or more further conductors might be dedicated to carrying frequency programming control signals from the control section to the receiver frequency synthesizer in the RF section, one or more still further conductors might be dedicated to carrying control signals from the control section for programming the transmitter frequency synthesizer, etc.).
Although such dedicated parallel conductor-type interconnection arrangements work, they have certain disadvantages. One disadvantage of such prior art interconnection arrangements is the relatively high cost. Multipin connectors and associated cables are expensive. Moreover, since radio frequency signals and digital control signals are not compatible with one another, precautions must be taken to minimize RF currents flowing on each such conductor. Each such conductor had to be "RF decoupled" at both the RF section end and the control section end using RF shunting and/or bypass networks (e.g., series-connected RF chokes and shunt-to-ground decoupling capacitors). Since each individual conductor had to include an RF decoupling network at each end, RF decoupling added significantly to the cost and complexity of the base station.
In addition, such parallel dedicated conductor interconnections created reliability and service problems. Reliability problems are created whenever an additional mechanical-type electrical connection is introduced. Such connections can corrode or otherwise mechanically deteriorate, degrading or destroying critical interconnections between the RF section and the control section. Preventive maintenance efforts had to be expended to ensure such connections were properly maintained. As the number of conductors increased, the complexity of testing for and isolating base stations faults also increased (thus increasing down time and service time).
Still further complexity is introduced by the requirement that most RF base stations must supply their customers with a wide variety of different base station options. The Federal Communications Commission authorizes base station operation on a user-by-user basis. Such authorizations specify different operating frequencies for different users, and may typically also specify different RF output powers for different users. One user may be authorized to operate with only, say, 100 watts of RF power, where as another user may be authorized to operate with several hundred watts of output power (different antenna configuration can also affect the power output required by a base station transmitter). The user needing only relatively low output power should not have to buy a base station having relatively high RF output power capability, since the high-powered components are generally more expensive and drastically increase the cost of the base station. Thus, for marketing and other reasons, base station manufacturers found it necessary to provide different RF output power options for their base stations. Similarly, different users may be assigned by the FCC to operate on completely different bands within the RF spectrum. RF circuitry designed for operation on a relatively low (e.g., several hundred megahertz) RF frequency is not capable of operating at high (e.g., 800 or 900 MH) RF frequencies. Accordingly, to meet the needs of a wide variety of users, a base station manufacturer must provide different base stations for different operating bands and for different power output levels.
Perhaps the most economic way for a base station manufacturer to meet such a wide variety of customer needs is to make different RF sections for different RF output powers, frequencies of operation, etc., and to make his control section mostly generic with respect to all such different RF sections. Thus, each of the RF sections can be made to be "plug compatible" with the same control section. While such an arrangement is both possible and practical, it introduces further complexities. Providing such a generic parallel interconnection interface so that the control section may interface with any RF section usually requires the control interface to provide a set of conductors that is a superset of conductors needed by any specific section. For example, unless very carefully designed so that all of the RF sections receive and provide the same control and status signals, some RF sections will not use some of the parallel connections provided to it. In order to accommodate the many different system configurations that are possible, the amount of I/O became large and very difficult to maintain and understand. Moreover, such careful design to provide generic parallel dedicated conductor interface is difficult and expensive and also may hamper further system expansion. Once the interface has been designed and manufactured, it is virtually impossible to add additional signal lines (e.g., to add further options or capabilities in response to customer demand) without significant redesign and remanufacturing efforts. Also, since the I/O lines were dedicated and implemented in hardware, implementation was costly and changes were very difficult.
Of course, much work has been done in the field of digital signal communications. For example, local area networks (LANS) are used throughout the world to link computers together. In addition, serial digital communications protocols and conventions have become relatively standardized. As one example, the pervasive standardized RS-232C serial digital signal interface is commonly used to connect a digital processor to another digital processor or to a peripheral.
Moreover, some microprocessor manufacturers have begun providing LAN software and associated hardware interfaces on-chip. As one example, the Intel 80C152 microprocessor includes a Global Serial Channel (GSC) which is a multi-protocol high performance serial interface targeted for data rates up to two megabits per second with on-chip clock recovery. The 80C152 implements the Data Link Layer and the Physical Link Layer as described in the ISO reference model for open systems interconnection. The GSC provided on Intel's 80C152 was optimized to implement the Carrier Sense Multi-Access with Collision Detection (CSMA/CD) protocol, and was designed specifically to allow standard baud rates (such as the proposed IEEE 802.3 LAN Standard 1.0 MBps). The Intel 80C152 was thus designed to make it possible to implement a LAN by merely more or less directly interconnecting 80C152 microcontrollers together (using appropriate transceiver ICs to transmit and receive the serial data).
Digital signal serial links are not unknown in the world of RF systems. For example, it is generally known to interconnect a digital controller with RF components within a mobile radio transceiver using a serial communications link. See, for example, U.S. Pat. No. 4,903,262 issued 20 Feb. 1990 entitled "Hardware Interface and Protocol for A Mobile Radio Transceiver"; and copending divisional application thereof Ser. No. 07/449,790 filed 15 Dec. 1989 now U.S. Pat. No. 5,109,543. See also U.S. Pat. No. 4,590,472 to Benson et al; U.S. Pat. No. 4,636,791 to Burke et al; and U.S. Pat. No. 4,684,941 to Smith et al. In addition, commonly assigned U.S. patent application Ser. No. 07/532,164 filed 5 Jun. 1990 now U.S. Pat. No. 5,175,866 entitled "Fail-Soft Architecture for Public Trunking System" describes a trunked RF repeater arrangement wherein various "trunking cards" each controlling an RF transceiver are linked together and with a dispatch console via a common "backup serial link." Base stations are now available that use multiple and dedicated relatively low-speed I/O for RF control, but such I/O arrangements are relatively inflexible and have no facilities for inter base station communications.
The present invention solves many of the problems mentioned above by providing a generic digital control signal link for communicating digital control signals within and between RF base stations. This new arrangement for communicating control signals within and between base stations practically reduces the number of separate interconnections between base station components, standardizes the way such components communicate with one another, allows single-point monitoring of an entire system (e.g., comprising one or many RF base stations), increases reliability, and reduces hardware complexity and cost.
The generic digital signal link provided by the present invention also eliminates the use of multiple lines for respective functions by using the same lines for various different functions depending upon requirements. By using such a generic digital signal link for inter and intra base station communications, RF and auxiliary control become standardized--reducing complexity in the product, in the documentation and in field repair techniques.
In the preferred embodiment in accordance with the present invention, the generic digital signal link is implemented using a multipoint communication architecture (e.g., EIA RS-485) with a Local Area Network (LAN) communications protocol. Many system components may be connected to the same link with no impact on system hardware performance. This permits the same link used for control interface between components (e.g., the control section and an RF section) within a base station to also be used for communicating with components and systems external to the base station. The generic link provides virtually unlimited expansion capabilities e.g., (only software changes are required to adapt the system for a completely new system component) and thus provides an expandability never before available in RF base station architectures.
In the preferred embodiment, the generic digital signal link is used for programming transmitter and receiver local oscillator synthesizers and is also used to transmit fault status signals from the RF section to the control section. This same link is additionally used to permit the control section to control the transmitter power amplifier output level. Due to its expandability, the very same generic digital signal link can be used to interface and interconnect with various trunking system components within the base station (e.g., voter system components, voice guard system components, auxiliary receivers, etc.). In addition, the very same generic digital signal link can be used to interconnect multiple base stations together (e.g., to implement a distributed control architecture or to otherwise provide centralized or distributed control capabilities) and/or to an external control and/or monitoring facility (e.g., a single-point monitor for overall system diagnostic, fault and operating parameter monitoring). While processors or other digital signal circuitry within various LAN nodes may perform significant processing if desired, in at least one configuration most such processors may provide mostly LAN communications support--thus simplifying system software design.